Tight binding model for a molecular nanowire embedded in between the metallic electrodes is used to derive the analytic expressions for a nonresonant tunneling current mediated by the localized and delocalized molecular orbitals associated with the terminal and regular interior units, respectively. The specific regimes of charge transmission controlled by resonant and nonresonant participation of terminal’s energy levels, are studied and the role of energy position of delocalized orbitals in formation of a distant superexchange electrode-electrode coupling , is clarified. The criteria for reduction of superexchange model of charge tunneling tothe flat barrier model are formulated and the parameters of barrier model (energy gap and effective electron mass) are specified in the terms of inter-site couplings and energy distance from the Fermi level to the delocalized wire’s HOMO level.
Special attention is payed to a derivation of analytic expressions for the tunneling current with use of different approximations. It has been shown that at passive participation of terminal units in a charge transmission process, the best correspondence with the observed current-voltage characteristics is achieved with use of the simplest Gauss and the mean-value analytic forms. This is supported by comparison of the theory with experimental results concerning the dependence of nonresonant tunneling current on the voltage bias and the number of chain units in alkanedithiol chains